Communication system



Sept. 24, 1946.

L. A. MEACHAM COMMUNICATION SYSTEM Filed July 23, 1943 T 20R vRu M 5 Wm M A L ATTORNEY Patented Sept. 24, 1946 UNITED. STATES PATENT OFFICE Bell Telephone Laboratories, Incorporated, New York, Y., a corporation of New York Claims, 1

This invention relates to telephone systems and particularly to intercommunicating systems in which a plurality of carrier channels are provided in a common transmission medium extending to all of the telephone stations in the system.

Objects of the invention are the improvement of intercommunicatingsystems comprising a plurality of carried channels in a common transmission medium extending to all of thetelephone stations and particularly in the provision of. a system in which privacy is assured and in which a busy signal device is operated at a calling station if the called station is already connected in communication with a third station. J

The invention is an improved intercom-municating system in which each of the telephone stations is assigned a different, pair of carrier chan-, nels in a common transmission medium extend.- ing to all of th stations, one of this pair of channels being used as a receiving channel, and the other as a transmitting channel on calls incom ing from any other station, and in which each of the stations is arranged to select the pair of channels assigned to any other station with which communication is desired, According to, a feature of the invention, means are provided for giV-' ing a busy signal to a calling station whenever the station with which communication is desired is busy, both when the last-mentioned station is busy in communication with a calling station and when the last-mentioned station is busy in communication with a called station.- According to another feature of the invention, meansare provided at each station for disabling a calling station when attempting to establish communication with another station which is then in communication with a third station.

Other features will be apparent from the fol lowing description of the system disclosed in the drawing, which system constitutes one embodiment of the invention. The invention is not limited, however, to the system and detailed arrangements shown in the drawing but is-generally applicable to carrier intercommunicating systems;

Referring to the drawing, which consistsof a single figure, the common transmission medium CM interconnects a, plurality of stations including stationsA, B and X. The station A is shown in detail and each of the'other stations is similar thereto. The transmission medium CM may.

. Application July 23,1943, Serial No. 495,867

The conductors may extendibetween all suiciscribers stationsin tandem or be extended to the.

various stations separately fromone or more branch points, thereby to secure optimum economy and flexibility. The direct current power for the subscribers stations is supplied, over the aforementioned conductors, from a common battery 5 which may be located at one of the stations or at any intermediate point. The common medium affords a plurality of channels, each of adifierent frequency; and each station in the exchange or intercommunicating system is individ- ,ually assigned two channels, one for receiving and the other for transmit-ting speech and signals on calls initiated at any other of the stations. When a station is i'dleand also when it is tion is arranged so that it is necessary for thebe any pair of conductors l and 2, for instance a,

balling subscriber to test thecommon transmission medium for the presence of a carrier of the frequency of the individual transmitting channel of the station with which'conversation is desired,- before originating the call. If carrier current of this particular frequency is already present, a busy signal is given to the calling subscriber and the calling subscriber's station is rendered inop-= erative to establishv a conversational connection with the busy station. v

The station A comprises a telephone handset which is normally positioned in a supporting era-- dle, station keys, one for each of the other sta-' tions, an oscillator-modulator tube for generating and modulating the carriers which are used,' to, transmit speech from station A to the other stations, and an amplifier tube coupling the output circuit of tube 80 to the conductors I and 2; The oscillator-modulator tube 80 iscontrolled by the transmitter l1 and by two tuned circuits, one of which comprises coil II and condenser 13 and is tuned by the operation of any one of the station keys to generate a carrier of the frequency of the receiving channel individually assignedto the corresponding station for transmitting: speech from station A to the called station. The other of these tuned circuits comprises coil 12 and condenser 14 and is tuned to generate a carrier dividually assigned to station A and used to transmit speech to any other station when station A is the called station. The tube 80 is also used to generate a carrier of the transmitting frequency individually assigned to station A, when station A is busy as a, calling station, thereby to give a busy signal at any other station at which the station A key is actuated to initiate a call to station A. Thermistor elements 15 and 16 constitute amplitude limiters in the oscillator circuit. Reference may be had to my Patent No. 2,303,485, granted December 1, 1942, for a detailed description of similar oscillator circuits for generating a plurality of frequencies.

The station A further comprises a transformer.

I and detector-amplifier tube 5 for receiving speech modulated carrier currents incoming from any other station with which station A is in communication, the receiver l6 being connected in the output circuit of tube E5. The condenser I4 and primary winding of transformer it and the secondary winding of transformer It and condenser l'2 constitute tuned circuits which, when all station keys are normal, are tuned to the frequency of the receiving channel individually as signed to station A. Station A further comprises a ringer 6-2 for giving an audible signal when station A is idle and is being called by another station, a busy lamp 63 for giving the subscriber at station A a busy signal if the station with which a conversational connection is desired is busy, and a busy relay 65 for rendering the receiving and transmitting circuits of station A ineffective for communication in case the handset is removed from the cradle while a station key is depressed andthe called station is busy. Station A further comprises a gas-filled discharge tube 6| for controlling the ringer 52, lamp 63 and relay 55, and comprises a power filter consisting of retard coil 90 and condenser $3 for supplying power to station A. While the telephone handset is in normal position in the supporting cradle, the contacts I'9 are open and the filaments of tubes i5, 80'- and B5 are deenergized.

Only two station keys are shown in the drawing, key 36 for operation on a call to station E and key M) for operation on a call to station X. As many more keys are provided as there are other stations. While the telephone handset is in normal position in the supporting cradle, any of the station keys may be operated; but removal of the handset actuates a locking mechanism which lockany key which was operated at the time of removal and which prevents the operation of any key subsequent to removal of the handset. This locking mechanism comprises a locking plate 24' having openings 35. .6 through which the plungers of the various keys project. The plate is supported horizontally between bearings 25 and is movable therein under the control of an actuating element 20 which engages the bent over end of the plate. The actuating element 28 is pivoted at 23 and, while the handset is in the cradle, the horizontal portion. of element 2 is depressed against the compression of spring 2! against the stop 22; and the locking plate is held in its ex treme left-hand position in which the locking col laws 35,. 45 on. plungers 34, 44 are in alignment withthe openings 35, 45 so that any one of the station keys may be actuated. The actuation. of any station key closes the associated contact springs and also depresses the common element 50 to actuate the contact springs 55, the element 5|]v being pivoted at 5.4 and normally held by spring 5-l against the stop 52. When the handset i out of the cradle, the cradle supporting element it effects closure of contacts l9 and the sprin 21 rotates the actuating element thereby to move the locking plate 24 to its extreme right position; in which position, operation of any station key is impossible due to the engagement of its locking collar with the top surface of thelocking plate. Any key, which is depressed at the time the handset is removed from the cradle, is locked in its operated position, by the engagement of its looking collar with the lower surface of plate 22, until the handset is replaced. Thi key locking mechanism illustrates the desired control, which con trol may be efiected by any suitable mechanism.

.Assume now that station A is idle, with the handset in the cradle and all station keys normal. Under this condition, the detector is tuned. to the frequency of the receiving channel which is individual to station A. At thi time the oscillator-modulator 8G .is tuned to generate a carrier of the frequency of the transmitting channel which is individual to station A, the tuned circuit comprising coil 1 i' and condenser 13 being short-circuited by the upper contact of springs 55. Assume further that a personat station A desires to communicate with someone at station E. To this end, key 30- is operated prior to removal of the receiver to determine whether or not the called station is busy. The depression of key 38 actuates contacts 31, 38,139 and 55. The closing of' contact 37 connects condenser 3i in parallel with condenser H and connects condenser 32 in parallel with condenser 52 thereby to tune the receiving circuits and detector of station A to the frequency of the transmitting channel individual to the called station B. The actuation of springs 55 opens the short-circuit across the tuned circuit comprising coil H and condenser 73, and the closing of contact 3% connects condenser 33 parallel with condenser #3 thereby to tune the oscillator 88 to generate a carrier current of the frequency of the receiving channel individual to station B. The actuation of springs 55 also opens the normally closed circuit path through ringer 62 and connects the winding of busy relay 55 and lamp 63 to conductor 92. If station E is busy when key 38 isoperated, carrier current of the frequency of the transmitting channel individual to station B is being transmitted therefrom over the common medium CM, and, since the receiving network comprising transformer it? and condenser I I' has been tuned to this frequency, a sufficiently high potential is thereby applied through resistor to the control element of tube 6! to cause breakdown of the tube and energization of busy lamp 53' and busy relay 55. The subscriber at station A will thereupon release the key as without removing the handset from the cradle. If, however, the subscriber ignores the busy lamp signal and removes the handset, the subscriber at station A is unable to communicate with station B because relay 65 holds open the receiver circuit and the filament circuit through resistor 54. If station 13 is idle, lamp 63 is not lighted and relay 55 is not operated; and the calling subscriber thereupon proceeds to remove the handset from the cradle while holding key 3!! depressed, thereby locking key 30 in its operated position until the handset is replaced. The actuation of contact 19 closes the filament energizing circuit through resistor 64 and the actuation of springs 55 opens the normally closed short-circuit across condenser 1.3; and with condenser 33 connected in parallel withcondenserlii, tube 89 generates carrier current of the frequency of thereother station at which the station A key may thereafter be operated, to indicate to any other subscriber who may wish to communicate with station A, that station A isbusy and to prevent any'such subscriber from communicating with station A and to preventany such subscriber from hearing the conversation betweenstations A and B. When conversation with station B is completed, the handset is replaced in the cradle thereby opening contact If! to deenergize'tubes I5, 80 and 85; The replacing of the handset also restores elements I8 20 and 24 to normal position thereby unlocking key and the return of key 30 to normal opens contacts 31,38 and 39 and efiects'the restoration of springs 55 to normal position; Since carrier current of the frequency of the transmitting channel individually assigned to. station A is no'longer being transmitted therefrom, any other station may now establish communication with station A by depression of the corresponding station key and removal of the handset from normal position.

.When the handset and station keys at station A are normal, the ringer 62 is connected to one or the two main electrodes of the gas-filled discharge tube 6|, the control element of tube 61 being connected through the primary winding of transformer I!) to conductor l of the common trans mission medium and through-condenser 'H to conductor 2 of thi medium. With none of the station keys operated, the'primary winding of transformer land condenser H are, tuned to the frequency of the receiving channel individually assigned to station A. When carrier current of this frequency is transmitted over conductors and 2 from some calling station, responsive to operationof the key corresponding to station A and removal of the receiver, the impedance offered by the primary Winding of transformer H! and condenser l I to current of this particular frequency is low enough to cause to be impressed on the control electrodes of discharge'tube 6| a highenough voltage to effect the breakdown of tube 61. The voltage similarly and simultaneously impressed on the control electrodes of the discharge tubes at other idle stations is insufficient to effect the breakdown of the discharge tubes thereat because the corresponding transformer windings and condensers at each of these stations offer a high impedance to current of the frequency of the receiving channel individually assigned station A. When tube 6| breaks down, the ringer 62 is actuated in a circuit traced from the positive pole of battery 5, conductor 2, right winding of retard coil 90, conductor 92, lower normally closed contact of spring- 55, windings of ringer 62, main electrodes of tube 6|, conductor 66, filaments of tubes 85, 83 and i5 in the order named, conductor 9|, left winding of retard coil 90 *and conductor l to the negative pole ofbattery 5;", The filaments of' the tubes 85, 8!! and 15. are .energized bythe current in this circuit to prepare forthe reception and transmission of speech currentsQWhen ringern62 is energized'it.

closes a shunt circuit from conductor 92, through resistor 64, and front contact of ringer 62 to'conductor 66, thereby causing the deenergization of tube Bl. to interrupt the energization of ringer 62. As soon as the ringer 62 deenergizes, 'the shunt circuitth'roughresistor 64 is opened and tube GI again breaks down to 'reenergiz'e ringer 62. Thus ringer 62 is alternately energized and deenergized to give an audible calling signal as long as carrier current of the receiving frequency of station A is being transmitted over the common medium CM and the handset at station A is in normal position. When the handset at station A is removed from the cradle to answer the call, a shunt circuit is closed by contacts l9 through resistor 64 thereby effecting the deenergization of tube 6| andpreventing its reenergizationwhile a conversational connection is maintained between station A and the calling station. While'th'e handset is removed at station A, the oscillator-modulator tube 80 generates current of the frequency of the transmitting channel assigned individually to station A, this current being modulated by speech, amplified by tube .85 and transmitted over conductors and 2 to the calling station. If the station key corresponding to station A is depressed at another station while station A is busy on a call from a first one of the other stations, the carrier current "being transmitted from station A will effect the energization of the busy lamp and busy relay at this other station and prevent the subscriber thereat from interfering withor hearing the conversation between station and said first other station. When the handsets are restored at both the calling station and at station A, station A is rendered selectable for communication with any other calling station.

What is claimed is:

1. In a telephone system,'subscribers stations,

1 a wide band transmission medium interconnecting said stations and constituting a plurality of channels each of a different particular frequency, a pair of said channels being individually assigned to each of said stations, one channel of the pair assigned to each station being used for transmittingspeech to and the other channel of the pair being used for receiving speech from any other calling station, receiving means and transmitting means at .each of said stations, the transmitting'means at each station comprising a carrier. source of the frequency of the assigned in-' dividual transmitting channel and a carrier source of variable frequency, means at each station normally tuning the receiving means there at to the frequency of the assigned individual receiving channel and normally tuning the variable carrier source thereat to the frequency of the assigned individual transmitting channel, means at each station for tuning the carrier source of vari-' able frequency thereat to the frequency of the receiving channel assigned individually to any other station with which communication is desired and for tuning the'receiving means thereat to the frequency of the transmitting channel assigned individually to any other station with which communication is desired, and signal ,means at each station normally responsive to carrier current of the frequency of the assigned individual receiving channel. 7 2. In a 'telephone'system, subscribers stations, a wide band transmission medium. interconnecting said stations'and constituting a plurality of carrier channels each. of a different frequency,-

eachof said stations having individually' assignedthereto; for. use on a, call from any other of said" stations, one of said channels as a, receiving:

channel and another-of said channels as a transmitting channel, receiving means at each of said stations normally tuned to the frequency of its assigned individual receiving channel, transmitting means at each of said stations comprising a carrier source tuned to the frequency of the assigned individual transmitting channel and a carrier source tunable to the frequency of any one of the receiving channels assigned individually to the other stations, and signal means at each of said stations normally responsive to carrier current of the frequency of the assigned individual receiving channel.

3.v In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a different frequency, each of said stations having individually assigned thereto, for use on a call from any other of said stations, one of said channels as a receiving channel and another of said channels as a transmitting channel, receiving means at each'of said stations normally tuned to the frequency of its assigned individual receiving channel, transmitting means at each of said stations comprising a carrier source tuned to the frequency of the assigned individual transmitting channel and a carrier source tunable to the frequency of any one of the receiving channels assigned individually to the other stations, signal means at each of said stations normally responsive to carrier currentof the frequency of the assigned individual receiving channel, other signal means at each of said stations for responding to carrier current of the frequency of any one of the transmitting channels individually assigned to the other stations, and means at each of said stations for tuning said other signal means to the frequency of the transmitting channel which is individual to any particular one of the other stations with which communication is desired.

4. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a different frequency, each of said stations having individually assigned thereto, for use on a call from any other of said stations, one of said channels as a receiving channel and another of said channels as a transmitting channel, receiving means at each of said stations normally tuned to the frequency of its assigned individual receiving channel, means at each of said stations for tuning the receiving means thereat to the frequency of the transmitting channel individual to any other station with which communication is desired, transmitting means at each of said stations comprising a carrier' source tuned to the frequency of the assigned individual transmitting channel and a carrier source tunable to the frequency of any one of the receiving channels assigned individually to the other stations, and signal means at each. of said stations normally responsive to carrier current of the frequency of the assigned individual receiving channel.

5. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a different frequency, each of said stations having individually assigned thereto, for use on a call from any other of said stations, one of said channels as a receiving channel and another of said channels as a transmitting channel, receiving means at each of said stations normally tuned to the frequency of its assigned individual receiving channel, means at each of said stations for tuning the receiving means thereat to the frequency of the transmitting channel individual to any other station with which communication is desired, transmitting means at each of said stations comprising a carrier source tuned to the frequency of the assigned individual transmitting channel and a carrier source tunable to the frequency of any oneof the receiving channels assigned individually to the other stations, signal means at each of said stations normally responsive to carrier current of the frequency of the assigned individual receiving channel, other signal means at each of said stations for responding to carrier current of the frequency of any one of the transmitting channels individually assigned to the other stations, and means at each of said stations for tuning said other signal means to the frequency of the transmitting channel which is individual to any particular one of the other stations with which communication is desired.

6. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constitutin a plurality of carrier channels each of a different frequency, each of said stations having individually assigned thereto, for use on calls from another of said stations, one of said channels as a receiving channel and one of said channels as a transmitting channel, a telephone receiver at each of said stations, means at each station comprising demodulating means connecting the receiver thereat to said common medium, a busy relay and a busy signal at each station, control means at each station connected to said common medium for control ling the operation of the busy relay and signal thereat, the control means at each station normally operatively tuned to the frequency of the assigned individual receiving channel of the station, keys at each station, one for each other station, means at each station rendered effective by the operation of any one of the keys thereat for operatively connecting the busy relay and busy signal thereat in circuit with the control means thereat, and means individual to each of said keys at each station for tuning the connection between the common medium and the control means thereat to the frequency of the transmitting channel assigned individually to the station corresponding to any operated key.

'7. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a different frequency, each of said stations having individually assigned thereto, for use on calls from another of said stations, one of said channels as a receiving chan-- nel and one of said channels as a transmitting channel, a telephone receiver at each of said Sta-- tions, means at each station comprising demodulating means connecting the receiver thereat to said common medium, means at each station normally tuning the demodulating means thereat to the frequency of the assigned individual receiving channel, a busy relay and a busy signal at each station, control means at each station connected to said common medium for controlling the operation of the busy relay and signal thereat, the control means at each. station normally operatively tuned to the frequency of the assigned individual receiving channel of the station, keys at each station, one for each other station, means at each station rendered effective bythe operation of any one of the keys thereat for op-- eratively connecting the busyrelay and busy sig-} nal thereatin circuit with the control means thereat, means individual to'each of said keys at :each station for tuning the connection between the common medium and th control means thereat to the frequency of the transmitting channel assigned individually to the station correspondingto any operated key, and contact means at each station actuated by the energization of said relay thereat, when the station corresponding to an operated one of the keys is'busy, for disabling the receiver thereat 8. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a difierent frequency, each of said stations havin individually assigned thereto for use on calls from another of saidstations one of said channels as a receiving channel and one of said channels as a transmitting channel, a telephone receiver at each station, means at each station comprising demodulating means connecting the receiver thereat to said common medium, means at each station normally tuning the demodulating means thereat to the frequency a of the assigned individual receiving channel, a busy relay and a busy signal at each station, control means at each station comprising a gas-filled discharge tube connected to said commonmedium for controlling the operation of the busy relay and busy signal thereat, the control means at each station normally operatively tuned to the frequency of the assigned individual receiving channel of the station, selective means at each station individual to each of the other stations for operatively connecting the busy signal and busy relay in circuit with the control means thereatand for tuning the connection between the discharge tube thereat and common medium to the frequency of the individual transmitting channel assigned to any other station with which communication is desired, thereby to enable the energization of the busy signal and busy relay at a calling station responsive to carrier current of the frequency of the individual transmitting channel of the station with whichcommunication is desired.

9. In a telephone system, subscribers stations, a wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a diiferent frequency, each of said stations having individually assigned thereto for use on calls from another of said stations one of said channels as a receiving chanof the busy relay and busy signal thereat, the control means at each station normally operatively tuned to the frequency of the assigned individual receiving channel of the station, selective means at each station individual to each of the other stations for operatively connecting the busy signal and busy relay thereat in circuit with the control means thereat and for tuning the connection between the discharge tube thereat and common medium to the frequency of the individual transwith whichcommunication is desired, thereby to effect the energization of the busy signal and busy relay at any calling station responsive'to carrier current of the frequency of the individual transmitting channel of the station with which communication is desired, and means at each station for transmitting a carrier of the frequency of the individual transmitting channel whenever the station is busy.

10. On a telephone system, subscribers stations, a Wide band transmission medium interconnecting said stations and constituting a plurality of carrier channels each of a different frequency, each of said'stations having individually assigned thereto for use on calls to said station from any other station one of said channels as a receiving channel and one of said channels as a transmitting channel, a source of power common to said stations and connected thereto by the conductors of said common transmission medium, each of said stations comprising a telephone receiver and transmitter, means comprising -a detector tube connecting said receiver to said common medium, a ringer, a busy lamp, a busy relay, means comprisin a gas-filled discharge tube connecting said ringer, lamp and relay to said common medium, means normally tuning the connection 7 between said detector tube and common medium to the frequency of the receiving channel individual thereto, means normally tuning the connection between said discharge tub and common medium to the frequency of the receiving channel individual thereto thereby to effect the breakdown of said tube responsive to the transmission over said common medium of carrier current of the frequency of said receiving channel to operate said ringer, an oscillator-modulator tube; means in the oscillatory control circuit of said oscillator-modulator tube tuned to the frequency of the individual transmitting channel, other means in the oscillatory control circuit of said oscillator-modulatortube tunable to the frequency of the individual receiving channel of any of the other stations, contacts normally short-circuiting said other means, means comprising an amplifier connecting the output of said oscillatormodulator tube to said common medium, means connecting said transmitter to said oscillatormodulator tube to modulate the oscillatory current generated in said tube, circuit means including said power source rendered efiective by the operation of said ringer and by the removal of the receiver from normal for energizing said detector and amplifier and oscillator-modulator tubes, calling keys, one for each of the other stations, means individual to each of said keys and rendered effective by operation of the key to tune the connection between said common medium and the detector tube and the connection between said common medium and'said discharge tube to the frequency of the transmitting channel individual to the station corresponding to the operated key and to tune said other means in the oscillatory control circuit of said oscillator-modu lator tube to the frequency of the receiving channel individual to the station corresponding to the operated key, means responsive to the operation of any of said station keys for opening the operating circuit of said ringer and for connecting said lamp and relay for operation responsive to transmission over said common medium of carrier current of the frequency of the transmitting channel individual to the station corresponding to the operated key, means actuated by the ener- 12 atingsaid locking mechanism responsive to the removal of the receiver from norma1 position to prevent the operation of any of said keys While said receiver is elf-normal and to lock operatedany key which is operated at the time of removal of the receiver from normal.

LARNED A. MEACHAM. 

